1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method and, in particular, to an image reading method in which an image is read by photoelectrically converting incident light from the image in units of single pixels when the image to be read has been divided into a plurality of pixels and to an image reading apparatus in which the above image reading method can be applied
2. Description of the Related Art
Conventionally, an image scanner is known in which an image is read (i.e. image data representing density values of each pixel in an image) in the following manner. Light emitted from a light source and transmitted through an image recorded on a photographic film or the like is measured (photoelectrically converted) in units of single pixels by a charge accumulation sensor (for example, a CCD). Photometric signals output from the CCD through an electronic circuit constructed so as to include an amplification circuit are then amplified and the amplified photometric signals are converted into digital data by an A/D converter.
In this type of scanner, generally (in the first reading method), the amount of light from the light source is adjusted (the amount of light for each component color is adjusted for a color scan) so that photometric values obtained from incident light when no photographic film has been set in place substantially conform to the maximum photometric value and so that no saturation of the values occurs. The amplification factor of the amplification circuit for amplifying the photometric signals output from the CCD is also adjusted and, image reading is performed after the CCD charge accumulation time has been adjusted (this is sometimes adjusted for each component color in a color scan).
In the first reading method, the dynamic range DR of analog photometric signals output from the amplification circuit is found by
DR=Vsat/Vdrk
when Vsat is the maximum level and Vdrk the black level of the photometric signals. In order to read an image at a wider dynamic range, the black level Vdrk may be reduced and the maximum level Vsat increased, however, the black level Vdrk, in particular, is dependent on: (1) the dark current output from the CCD; (2) noise output from the CCD; (3) the drift of the amplification circuit; and (4) noise output from the amplification circuit.
Consequently, the above (1) to (4) are factors that inhibit the widening of the dynamic range when reading a photographic film. (1) and (3) out of the above (1) to (4) can be substantially removed by correcting the dark current (i.e. by correcting the level of the photometric signals by the amount of the difference between the ideal level of the photometric signals when reading optical black (normally 0) and the actual level thereof).
When dark current correction is performed, because the black level Vdrk is replaced by the noise level of the CCD and the amplification circuit Vnoi, the dynamic range of the photometric signals is found by
DR=Vsat/Vnoi
Accordingly, in order to widen the dynamic range of a reading in a scanner with the above structure, it is necessary to reduce (2) the noise output from the CCD and (4) the noise output from the amplification circuit in addition to performing dark current correction. Thus it is necessary to use a CCD having low noise and high performance and to design an amplification circuit also having low noise. The problem is thus that costs are high.
Moreover, when the analog section of a scanner having a CCD and an amplification circuit is designed to have a wide dynamic range, it is also necessary to use an A/D converter which separates and converts the level of input signals into multibit data as the A/D converter for converting photometric signals into digital data. However, the cost of the A/D converter increases the greater the number of multibits. In particular, when dealing with image data comprising a plurality of pixels such as that from an image scanner, high speed analog digital conversion is demanded. As a result, the analog digital converter ends up being extremely expensive. Accordingly, currently, the specifications of each section of an image scanner are determined so that the widest dynamic range possible under the constraints of cost is obtained. Consequently, the performance of the scanner (i.e. the photometric dynamic range and the image reading speed possible from the analogxe2x80x94digital conversion speed) is not always satisfactory.
Further, high performance negative scanners are also known which read negative images with a high level of accuracy by separating negative images recorded on a color negative film into a plurality of pixels (for example, 1000 pixels) and separating each pixel into each component color and measuring the light thereof in order to determine exposure conditions used when a photograph printer exposes the images onto a photosensitive material such as photographic paper or the like. In this type of high performance negative scanner, the light of each negative image is preliminary measured under photometric conditions in which it is certain that saturation will not occur (prescan) and the density of the lowest density pixel in the negative image is detected. A main photomeasurement (fine scan) is then performed in which the charge accumulation time of the CCD is adjusted for each of the negative images (adjusted for each component color in a color scan) so as to be the longest possible time without the output being saturated by the light from the lowest density pixels, thus ensuring the maximum dynamic range (second reading method).
In the second reading method, often the density of the lowest density pixel is comparatively high relative to, for example, an over exposed negative image which has high density. Therefore, the charge accumulation time for a fine scan is adjusted so as to be long. Moreover, often the density of the lowest density pixels is comparatively low (namely, is close to or identical to the film base density) relative to an under exposed negative image which has a low density. Therefore, the charge accumulation time for a fine scan is also adjusted so as to be short.
Because the gradient of the change in the density relative to the change in the exposure amount in a negative film is small (xcex3 less than  less than 1), the gradation of a negative image is a soft gradation and the contrast of the negative image is low. Moreover, because the above high performance negative scanner uses a CCD having a comparatively rough photometric point density (pixel density), the contrast of the light incident on the CCD from each pixel of the negative image becomes still lower. As a result, by adjusting the charge accumulation time in accordance with the density of the low density pixels, as in the second reading method, negative images of any state of exposure type (over exposed negative images/normally exposed negative images/under exposed negative images) can each be read at a wide dynamic range.
However, in the second reading method, reading negative images having high contrast at a wide dynamic range such as negative images of scenes photographed using reverse light, negative images photographed using strobe lighting, and negative images in which light sources are contained in the image is difficult. Moreover, the dynamic range of the reading is also insufficient when reading images recorded on reversal film which has a large gradient of the change in the density relative to the change in the exposure amount (xcex3≈1), or when making high accuracy readings of images which have been separated into a plurality of pixels (for example, several hundreds of thousands of pixels). This is because the contrast of the light incident on the CCD from each pixel of the image is extremely high.
The present invention has been achieved in order to solve the above problems. It is an object of the present invention to provide a low cost image reading apparatus and image reading method which make possible the reading of an image at a wide dynamic range.
In order to achieve the above objectives, in the image reading apparatus according to the first aspect of the present invention, there is provided: a reading apparatus for reading an image in units of single pixels, after the image to be read has been separated into a plurality of pixels, by photoelectrically converting incident light from the image; determination device for determining suitable reading conditions for the image for each pixel or for each of small areas comprising a plurality of pixels, based on the result of the image reading; and a control apparatus for performing, based on the result of the determination by the determination device, a control process so that output image data identical to the image data obtained if each pixel or each small area of the image were read under the reading conditions determined to be suitable for each is obtained from the results of the image reading by the reading apparatus.
The reading apparatus according to the first aspect reads an image in units of single pixels, after the image to be read has been separated into a plurality of pixels, by photoelectrically converting incident light from the image. Note that, the reading apparatus can also be structured so as to include, for example, a reading sensor provided with a plurality of cells which reads the image with each cell by photoelectrically converting incident light from the image to be read (for example, a charge accumulation type reading sensor which accumulates signal charges obtained by photoelectric conversion). Note also that the incident light from the image being read may be light transmitted through the image or light reflected from the image.
In the reading of the image by the reading apparatus, when the amount of incident light (alternatively, the integral value of the amount of incident light within a reading period) is too great compared to the sensitivity of the reading apparatus, the reading accuracy is decreased due to saturation of the photoelectric conversion output. When the amount of incident light is too small compared to the sensitivity of the reading apparatus, the reading accuracy is decreased due to the photoelectric conversion output being too small. Consequently, when considering the dynamic range of a reading, it is desirable that the reading conditions are controlled so that the amount of incident light is as large as possible without being so large as to cause saturation of the photoelectric conversion output. However, because the density values or luminance values of the image being read vary from pixel to pixel or from small area to small area, they also differ for each pixel in an image with regard to the suitable reading conditions.
To counter this, the determination device of the first aspect, determines suitable reading conditions for the image for each pixel or for each small area comprising a plurality of the pixels of the image, based on a result of reading the image. Note that the result of a preliminary reading of the image being read by the reading apparatus (known as a prescan) may be used for the above result of reading the image. Alternatively, when the determination device is structured so as to include an image reading apparatus separate to the reading apparatus, the result of reading the image by the image reading apparatus (prescan) may be used for the above result of reading the image. Moreover, as will be described in the tenth aspect, it is also possible to use the results when the image is read a plurality of times by a reading apparatus under different reading conditions.
Furthermore, the reading conditions can include at least one of a physical amount relating to the sensitivity of the reading apparatus (for example, the length of time of the reading by the reading apparatus (corresponding to the charge accumulation time in a charge accumulation type image sensor: even if the amount of incident light is constant, because the value of the output of the reading apparatus changes due to the length of time of the reading, the sensitivity of the reading apparatus appears to change)) and a physical amount relating to the amount of incident light. It is also possible to obtain the suitable reading conditions by calculating and setting values representing the suitable reading conditions for the image as the values of the above physical amounts.
The control apparatus of the first aspect performs, based on the result of the determination by the determination device, a control process so that output image data identical to when each pixel or each small area of the image is read under the reading conditions determined to be suitable for each is obtained from the results of the image reading by the reading apparatus.
The control process for obtaining the above output image data can be achieved by, in the second aspect, for example, controlling the reading apparatus such that, when the reading apparatus is structured such that the reading conditions can be varied between units of pixels or small areas comprising a plurality of pixels, the reading conditions for each pixel or for each small area during the image reading by the image reading apparatus each match the suitable reading conditions determined by the determining means. As a result, in a single image reading by the reading apparatus, the image being read is read under suitable reading conditions both for pixel units and for small area units. The results of the reading by the reading apparatus can be used as output image data.
The control process for obtaining the above output image data can also be achieved by, as is described in the tenth aspect, for example, selecting for each pixel or each small area data which corresponds to the most suitable reading conditions determined by the determining means from the image data obtained from each of the plurality of image readings made under different reading conditions by the reading apparatus, and synthesizing this as output image data. In this case, the output image data which is equal to that when the image being read is read under suitable reading conditions for both pixels units and small area units is synthesized from the results of the plurality of image readings by the reading apparatus.
In the method described above, because output image data which is equal to that when the image being read is read under reading conditions determined as suitable for both pixels units and small area units (reading conditions in which the amount of incident light is as large as possible without saturation of the photoelectric conversion output occurring) is obtained, output image data equivalent to the result of the image being read at a wide dynamic range can be obtained even in cases such as when the image being read has a high level of contrast.
Moreover, in the first aspect, because output image data which is equal to that obtained when the image is read under reading conditions determined as suitable for both pixels units and small area units is obtained by selecting reading conditions in pixel units or small area units, an image reading apparatus with the equivalent of the dynamic range necessary for reading the image can be constructed at low cost without it being necessary to construct the reading apparatus with high cost sections such as low noise reading sensors.
The image reading apparatus according to the second aspect of the present invention comprises: reading means which reads the image by photoelectrically converting incident light from the image in units of single pixels when the image to be read has been separated into a plurality of pixels and which is able to change the image reading conditions in units of pixels or in units of small areas each comprising a plurality of pixels; determination device which determines suitable reading conditions for the image for each pixel or for each small area comprising a plurality of pixels based on a result of reading the image; and a control apparatus for performing control such that the reading conditions for each pixel or each small area during the image reading by the image reading apparatus match the suitable reading conditions determined by the determination device.
In the second aspect, the reading apparatus is able to change the image reading conditions for units of single pixels or for units of small areas comprising a plurality of pixels. The determination device determines suitable reading conditions for each pixel or each small area based on the result of the image reading. The control means performs control processing such that the reading conditions for each pixel or each small area during the image reading by the reading apparatus match each of the determined suitable reading conditions. As a result, in the same way as in the first aspect, it is possible to read an image at a wide dynamic range and to construct the image reading device cheaply.
Note that the construction of an image reading apparatus capable of altering the reading conditions of an image in units of single pixels or in units of small areas comprising a plurality of pixels can be achieved by including in the image reading apparatus a reading sensor for reading the image by photoelectrically converting each pixel of the incident light from the image, and an incident light amount alteration apparatus capable of altering the amount of incident light striking the reading sensor in pixel units or in small area units.
The incident light amount alteration apparatus can be constructed from, for example, a transmission light amount adjustment device such as an LCD which is provided with a plurality of cells and which is capable of altering at each cell the amount of transmission light or, alternatively, from a reflection light amount adjustment device such as a DMD (digital micromirror device) which is provided with a plurality of cells and which is capable of altering at each cell the amount of reflection light. By corresponding these cells to pixels or small areas and controlling the amount of transmission light or reflection light of the devices at each cell, the amount of incident light striking the reading sensor can be altered in units of single pixels or in units of small areas.
When the reading apparatus has the above structure, the control by the control apparatus of the reading conditions can be achieved by independently controlling the amount of incident light striking the reading sensor via the incident light amount alteration apparatus in units of single pixels or in units of small areas. The effect achieved by the third embodiment is that there is no longer any need to use as the reading sensor of the reading apparatus a structurally complicated reading sensor such as a charge accumulation type reading sensor capable of independently altering the charge accumulation time for units of single pixels or units of small areas.
The construction of a reading apparatus capable of altering the reading conditions of an image in units of single pixels or in units of small areas comprising a plurality of pixels can be achieved, as described, for example, in the sixth aspect of the present invention, by including in the image reading apparatus a charge accumulation type reading sensor for reading the image by photoelectrically converting incident light from the image for each pixel and accumulating this as a charge, and capable of independently altering the charge accumulation time for pixel units or for small area units.
When the reading apparatus has the above structure, the control of the reading conditions by the control apparatus is performed by independently controlling the charge accumulation time of the reading time for pixel units or for small area units. According to the sixth aspect, although the structure of the reading sensor is complicated, the incident light amount alteration apparatus described in the third aspect is no longer a necessary part when controlling the image reading conditions for pixels units or for small area units each of which comprises a plurality of pixels, thus allowing the number of parts to be reduced.
Moreover, the image reading apparatus according to the present invention is structured such that light other than from the image being read is also incident on the reading sensor (for example, is structured such that, when the image being read is recorded on a recording medium such as a photographic film, light which has passed through or been reflected from regions other than the regions where the image is recorded on the photographic film is also incident of the reading sensor). In particular, when the amount of incident light other than from the image being read is greater than the amount of incident light from the image being read, if the reading sensor is, for example, a charge accumulation type reading sensor, then the incident light other than from the image being read has an adverse effect on the image reading, such as the charge accumulated in the reading sensor from the incident light other than from the image being read being saturated.
It is possible to prevent the incident light other than from the image being read having an adverse effect on the reading by, for example, shutting out incident light other than from the image being read using a mask or the like. However, as in the third aspect, in an aspect in which a second reading apparatus is constructed so as to have an incident light amount alteration means and a second control apparatus controls the reading conditions by controlling the amount of incident light incident on the reading sensor in units of pixels or small areas using the incident light amount alteration apparatus, then, as described in the fourth aspect, it is preferable that the second control apparatus controls the amount of incident light on the reading sensor in units of pixels or small areas using the incident light amount alteration apparatus such that the amount of incident light other than from the image being read from among the incident light incident on the reading sensor is below a predetermined value.
In contrast, in the sixth aspect of the present invention, in an aspect in which the second reading apparatus is structured so as to include a charge accumulation type reading sensor capable of altering the charge accumulation times for stand-alone pixel units or small area units, and the second control apparatus controls the reading conditions by controlling the charge accumulation time of the reading sensor in stand-alone units of pixels or small areas, then, as described in the seventh aspect, it is preferable that the second control apparatus controls the charge accumulation time of the reading sensor in units of pixels or small areas such that the charge accumulation time in the photoelectric conversion of incident light other than from the image being read from among the incident light incident on the reading sensor is below a predetermined value.
By controlling the charge accumulation time or incident light amount as described above, there is no need to make the structure more complex such as by providing a mask for shutting out incident light other than from the image being read and incident light other than from the image being read can be prevented from having an adverse effect on the reading of the image.
Further, irregularities in pixel units which are caused by the image reading apparatus are sometimes generated in the results of an image reading by the reading sensor. Examples of the causes of these irregularities are unevenness in the amount of light illuminating the image being read;
aberration in the optical system irradiating the light from the image onto the reading sensor; and irregularities in the sensitivity for each pixel of the reading sensor. Moreover, when the image being read is one that has been made visible by performing developing processing and the like on a photographed object which has been recorded on a photographic film by photography using a camera, density unevenness in the image being read is generated due to aberrations in the optical system of the camera. Therefore, aberrations in the optical system of the camera are also a cause of irregularities in pixel units in the results of an image reading by a reading sensor.
It is possible to avoid irregularities in pixel units in the results of an image reading by a reading sensor by, for example, performing a correction processing to correct the image reading results in units of each pixel.
However, as in the third aspect, in an aspect in which the second reading apparatus is constructed so as to include an incident light amount alteration apparatus and the second control apparatus controls the reading conditions by controlling the amount of incident light on the reading sensor in units of pixels or small areas using the incident light amount alteration apparatus, then, as described in the fifth aspect, it is preferable that the second control apparatus controls the reading conditions by controlling the amount of incident light on the reading sensor in units of pixels or small areas using the incident light amount alteration apparatus such that density unevenness in an image being read and irregularities in each pixel unit in the results of an image reading by a reading sensor caused by the image reading apparatus are corrected.
In contrast, as in the sixth aspect of the present invention, in an aspect in which the second reading apparatus is constructed so as to include a charge accumulation type reading sensor capable of altering the charge accumulation time for stand-alone units of pixels or small areas, and the second control apparatus controls the reading conditions by controlling the charge accumulation time of a reading sensor in stand-alone units of pixels or small areas, then, as described in the eighth aspect, it is preferable that the second control apparatus controls the reading conditions by controlling the charge accumulation time of a reading sensor in units of pixels or small areas such that density unevenness in an image being read and irregularities in each pixel unit in the results of an image reading by a reading sensor caused by the image reading apparatus are corrected.
By controlling the charge accumulation time or incident light amount as described above, it is possible to avoid irregularities in pixel units in the results of an image reading and there is no need to perform correction processing on the results of the image reading for each pixel unit.
Note that, in the third and fourth aspects, it is also possible, as is described in the ninth aspect, to construct the reading apparatus such that it contains a light amount adjustment apparatus capable of adjusting the amount of light of at least one of illumination light illuminating an image and incident light incident onto a reading sensor from an image. In this case, the control apparatus is able to control the reading conditions by controlling the amount of light of at least one of illumination light and incident light via the light amount adjustment apparatus.
The light amount adjustment apparatus may be formed from a diaphragm, a light reduction filter, or the like, and, generally, these optical parts are provided in the structure of an image reading apparatus. Accordingly, according to the ninth aspect of the present invention, by controlling the light amount via a light amount adjustment apparatus, it is possible to reduce the width of the alteration of the incident light by the incident light amount alteration apparatus described in the third aspect, or to reduce the width of the alteration of the charge accumulation time by the reading sensor described in the sixth aspect. In addition, an increase in the number of parts can be avoided by using a diaphragm or light reduction filter already present in the apparatus as the light amount adjustment apparatus.
The image reading apparatus according to the tenth aspect of the present invention comprises: a reading apparatus which is provided with a light amount adjustment apparatus capable of adjusting the amount of light of at least one of illumination light illuminated onto an image being read and incident light from the image, and which reads the image a plurality of times by photoelectrically converting incident light incident from the image in units of single pixels when the image has been divided into a plurality of pixels and also causes the reading conditions to be varied for each reading by adjusting the amount of incident light using the light amount adjustment apparatus; determination device which determines the most suitable reading conditions for each pixel or for each small area comprising a plurality of pixels from among the reading conditions for each image reading, based on image data obtained from each of the plurality of image readings by the reading apparatus; and a control apparatus which selects for each pixel or each small area data corresponding to the most suitable reading conditions determined by the determination device from the image data obtained from each of the plurality of image readings by the reading apparatus and synthesizes this as output image data.
The reading apparatus according to the tenth aspect reads the image a plurality of times by photoelectrically converting incident light incident from the image being read in units of each single pixel when the image has been divided into a plurality of pixels and also causes the reading conditions to be varied for each reading by adjusting the amount of incident light using the light amount adjustment apparatus. Note that it is possible to use an existing diaphragm, light reduction filter, or the like as the light adjustment apparatus of the tenth aspect as well.
The determination device determines the most suitable reading conditions for each pixel or for each small area comprising a plurality of pixels from among the reading conditions for each image reading by the reading apparatus, while the control apparatus selects for each pixel or each small area data corresponding to the above determined most suitable reading conditions from the image data obtained from each of the plurality of image readings and synthesizes this as output image data. As a result, because output image data equivalent to when the image being read is read under the suitable reading conditions for each pixel or small area unit is synthesized from the results of the plurality of image readings by the reading apparatus, it is possible to read the image at a wide dynamic range and to construct the image reading apparatus cheaply.
Note that when the reading conditions are varied by adjusting the amount of incident light, if, for example, the reading apparatus includes a charge accumulation type reading sensor, saturation of the accumulated charge amount occurs in at least a portion of the cells during the plurality of image readings. However, if a charge accumulation type sensor having anti-blooming characteristics (for example, a sensor having an overflow drain structure) is used, the overflow charge from the cell in which the accumulated charge saturation occurred can be prevented from having adverse effects, which is naturally preferable. Moreover, when the above reading sensor is used in the tenth aspect, the determination of the most suitable reading conditions can be made on the basis of whether or not saturation of the accumulated charge occurred in each cell.
In the image processing method according to the eleventh aspect of the present invention, suitable reading conditions for the image to be read are determined for each pixel or for each small area comprising a plurality of pixels. Then, based on the above determination results, control is performed such that output image data equivalent to that obtained when the image is read under the suitable reading conditions for each pixel unit or each small area unit is obtained from the results of reading the image by photoelectrically converting incident light from the image in units of single pixels when the image to be read has been divided into a plurality of pixels. Therefore, in the same way as in the first aspect, an image can be read at a wide dynamic range without any major increase in the costs being incurred.